Catalytic Hydrothermal Deoxygenation Of Fatty Acid To Green Diesel Without External Hydrogen Supply

In recent years,the problem of waste lipid treatment has become increasingly serious.China produces more than 5 million tons of waste lipid every year,but a large amount of them enters the circulation field through illegal channels,posing a huge threat to the environment and human health.The conversion of waste lipids into sustainable bioenergy—green diesel through hydrothermal deoxygenation process without external H₂ supply provides new strategy for solving these problems.Compared to traditional deoxygenation process,hydrothermal deoxygenation process has the following advantages:without external H₂ supply,the solvent water is environmentally friendly and inexpensive,the hydrolysis and deoxygenation of lipids are carried out in one-pot,and in situ H₂ production of glycerol,a by-product of lipid hydrolysis,to reduce H₂consumption.Therefore,it is of great significance to study the catalytic hydrothermal deoxygenation process without external H₂.This paper systematically studies the hydrothermal deoxygenation process of fatty acid,which is the main hydrolysate of lipids.First,the key factors for the hydrothermal deoxygenation of fatty acids were studied.Secondly,the synergistic hydrothermal deoxygenation of the co-reactants and stearic acid was studied.Finally,the mechanism of urea as a co-reactant to promote the hydrothermal deoxygenation of stearic acid was explored.The main work of this thesis is summarized as follows:First,the catalysts for the hydrothermal deoxygenation of stearic acid under N₂atmosphere were tested,and the order of their activities was:Pt/C>Ru/C>Pd/C>Rh/C.Among them,Ru/C has the lowest price and can convert stearic acid into a mixture similar to diesel components.The effects of catalyst dosage,reactor type,reaction time,initial headspace atmosphere,initial N₂ pressure,reactants on the hydrothermal deoxygenation of fatty acids are explored.The path of hydrothermal deoxygenation of lipids under Ru/C catalyzed is as follows:lipids are first hydrolyzed to generate fatty acids and glycerol.Unsaturated fatty acids are hydrogenated to generate saturated fatty acids,and then saturated fatty acids are converted to octadecane and heptadecane through hydrodeoxygenation and decarboxylation/decarbonylation respectively.After that,the long-chain alkanes are converted to short-chain alkanes and CH₄ via hydrogenolysis and cracking.At the same time,the glycerol produced by lipid hydrolysis undergoes aqueous phase reforming to generate H₂ for use in fatty acid hydrogenation and deoxygenation.Secondly,a method of synergistic hydrothermal deoxygenation of co-reactants and stearic acid was established,and the effect of various co-reactants in promoting the hydrothermal deoxidation of stearic acid was tested and its laws and mechanisms were explored.The study found that various co-reactants mainly exhibit two promoting effects in synergistic hydrothermal deoxygenation.One is to promote the deoxygenation and increase the yield of green diesel,and the other is to inhibit the cracking and methanation and reduce the yield of gas products.The mechanism of alcohols,carboxylic acids,and metals co-reactants to promote the hydrothermal deoxygenation of stearic acid is H₂production in situ,and the promotion effect is positively related to the theoretical maximum H₂ production.Phosphoric acid and urea do not produce H₂ in the hydrothermal reaction,but they increase the yield of green diesel while reducing the yield of gas products.Finally,the mechanism of urea to promote the hydrothermal deoxygenation of stearic acid was explored.The study found that dosages of urea have a greater impact on the synergistic deoxygenation reaction.When the dosage of urea is low,it will have an inhibitory effect on the deoxygenation of stearic acid.If the dosage is too high,the promotion effect of the deoxygenation reaction of stearic acid will not be improved significantly.A series of experiments finally proved that ammonia produced in-situ is the main reason that urea promotes the hydrothermal deoxygenation of stearic acid,and the ammonia content in the reaction system is obviously positively correlated with the yield of green diesel.It is speculated that the promotion mechanism is that ammonia is used as an electron donor to maintain the reducing conditions of the reaction system.